Combined hybrid and local dimming control of light emitting diodes

ABSTRACT

An LED backlight controller combines global/hybrid and local brightness/dimming control for an LED backlight illuminator with local regions illuminated by associated LED strings. Global/hybrid brightness/dimming control performs hybrid digital modulation control for a predefined lower range of brightness levels, with string current maintained at a substantially constant level associated with a predefined maximum brightness for the lower range (controlling brightness by adjusting digital modulation, such as PWM duty cycle, up to a maximum), and performs hybrid string current control for a predefined higher range of brightness levels (controlling brightness by adjusting string current). Local dimming control is performed by introducing a local digital modulation signal into a hybrid digital modulation control path for the associated string, so that digital modulation for the associated string is a combination of local digital modulation and global/hybrid digital modulation.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of the filing date of U.S.Provisional Patent Application Ser. No. 62/064,410, also entitledCombined Hybrid and Local Dimming Control for Light Emitting Diodes,filed Oct. 15, 2014, which is incorporated herein in its entirety byreference.

TECHNICAL FIELD

Aspects of the invention are directed, in general, to illuminationcontrol, and more specifically to, combined hybrid and/or local dimming(illumination) control suitable for use in (backlight) illumination foran (Liquid Crystal Display (LCD)) display, or the like.

BACKGROUND

Local dimming is used in a Light Emitting Diode (LED) backlight systemthat is configured to separately control backlight illumination anddimming for local display regions or blocks of an LCD display. LCDdisplay systems can be configured with full array or edge-lit backlightillumination systems, including configuration of the backlightillumination system to provide local illumination and dimming for localdisplay blocks in a full array, or local display regions or blocks in anedge-lit array. For example, local dimming can be implemented in an LCDdisplay with an LED backlight system configured into local displayregions or blocks. The backlight LEDs are positioned so that an LEDstring illuminates an associated local display regions or blocks. EachLED driver string may have a driver that outputs a separate brightnesssetting, controlling the illumination of its associated LED string.Local dimming control is used to enable dimming control of each localdisplay regions or blocks independent of dimming control applied to therest of the display. Hence, for local illumination and dimming, LEDs arepositioned so that an LED string illuminates a single local displayregion or block. Each LED (string) driver can be controlled for aseparate brightness setting based on local illumination and dimmingcontrol. LED driver Integrated Circuits (ICs) may employ multiplechannels and, with various dimming methods and on-chip protections. SuchICs may be somewhat application specific, adapted to providebacklighting control for a particular type of device, such assmartphones, tablet computing devices, notebook computers, computerdisplays, televisions, or the like.

SUMMARY OF THE INVENTION

Aspects of the invention provide combined hybrid and local dimmingcontrol of LEDs. In an LCD system, or other illuminated panel, withcontrolled LED dimming, a plurality of strings of backlight LEDs arearranged to provide backlight illumination to the panel, with eachcontrolled by an LED driver, and at least some LEDs arranged to providebacklight illumination to respective local display regions or blocks ofthe panel.

A local LED driver control unit may be configured to generate a hybriddigital modulation control signal in a digital modulation control pathassociated with a digital modulation setting corresponding to a desiredLED brightness value associated with a master brightness control signal,up to a specified maximum digital modulation setting, while maintainingstring current and string voltage substantially constant, for a range oflower LED master brightness values.

Meanwhile, for a range of higher LED master brightness values above theLED brightness value associated with the maximum digital modulationsetting, a hybrid current control signal is generated in a currentcontrol path associated with a string current corresponding to a desiredLED brightness value associated with the master brightness controlsignal.

Local dimming and illumination control for a LED driver associated witha LED string associated with a selected local display regions or blocksof the illuminated panel is selectively controlled by generating a localdigital modulation control signal in the digital modulation control pathassociated with a digital modulation setting corresponding to a desiredLED brightness value for the selected local display regions or blocks,and combining the local digital modulation control signal with thehybrid digital modulation control signal to generate a composite localdigital modulation control signal used to control the associated LEDdriver, the composite digital modulation control signal corresponding tothe desired LED brightness value for the selected local display regionsor blocks.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, wherein:

FIG. 1 is a diagrammatic illustration of an example environment whereinexamples of the present systems and methods for combined hybrid andlocal dimming control of LEDs may be employed, in accordance with someembodiments;

FIG. 2 is a graph (200) of (global) hybrid LED dimming as may beemployed as a part of the present systems and methods for combinedhybrid and local dimming control of LEDs, in accordance with someembodiments;

FIG. 3 is a diagrammatic, block illustration of at least a portion of anexample system or apparatus for employing the present combined hybridand local dimming control of LEDs, in accordance with some embodiments;and

FIG. 4 is a flowchart of an example implementation of the presentsystems and methods for combined hybrid and local dimming control ofLED, according to some embodiments.

DETAILED DESCRIPTION

The invention now will be described more fully hereinafter withreference to the accompanying drawings. This invention may, however, beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein. Rather, these embodiments areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the invention to those skilled in the art. Oneskilled in the art may be able to use the various embodiments of theinvention.

Combined hybrid and local dimming control in accordance with the presentsystems and methods is suitable for use in backlight control for anilluminated structure, such as an illuminated panel, for example an LCDdisplay, or the like. Hybrid dimming control comprises a hybrid, that isa combination of digital modulation, such as Pulse Width Modulation(PWM) and electrical current dimming (illumination) control, which maybe implemented as hybrid PWM dimming during a predefined range ofrelatively lower LED brightness levels in which PWM modulation may beused to control brightness (with string current substantially constant).For a predefined range of relatively higher LED brightness levels,hybrid current control may be used to adjust LED brightness (with PWMfixed at some maximum setting, such as one-hundred percent). Such hybriddimming control is described as “A system for Combined DigitalModulation and Current Dimming Control for Light Emitting Diodes,”published as U.S. Patent Publication 2012/0086701 on Apr. 12, 2012,which issued as U.S. Pat. No. 8,872,810 on Oct. 28, 2014, which iscommonly assigned, and names as an inventor the inventor hereof, and thedisclosure of which is incorporated herein in its entirety by reference.

For combined hybrid and local dimming control in accordance withembodiments of the present systems and methods hybrid (PWM and/orcurrent) dimming is applied to the full display, and local dimming forlocal display regions or blocks is implemented as local PWM modulationcontrol introduced into the hybrid PWM control path (without affectingstring currents or string voltages otherwise set by global (hybrid)dimming). That is, local dimming control is combined with the hybriddigital modulation output of the hybrid brightness/dimming control sothat dataflow for local dimming is the same independent of global/masterbrightness/dimming control (i.e., independent of the hybridbrightness/dimming range). Embodiments of combined hybrid and localdimming control according to the present systems and methods can beimplemented in full-array and edge-lit backlight systems, as well as inother lighting systems, such general illumination (lighting) systems,signage, etc.

Herein, description of example embodiments of combined hybrid and localdimming control according to the present systems and methods, refergenerally to PWM modulation as an example form of digital modulationcontrol for an LED driver, in contrast to current control that adjustsstring electrical current. Other embodiments of hybrid and/or localdimming control according to the present systems and methods can beimplemented with other forms of digital modulation.

FIG. 1 is a diagrammatic illustration of an example environment 100wherein examples of the present systems and methods for combined hybridand local dimming control of LEDs may be employed, in accordance withsome embodiments. For example, FIG. 1 shows deployment of the presentcombined hybrid and local dimming control of LEDs in conjunction withilluminable assembly 102, such an illuminable assembly may be a lightingpanel, (LCD) display, or the like. The present systems and methods forcombined hybrid and local dimming control of LEDs combine LED hybriddimming with local dimming, such as for backlight illumination controlof LCD display system 102. Therein, a plurality of strings of backlightLEDs (104, generally) are arranged to provide backlight illumination tothe LCD, each string controlled by an LED driver (106, generally), andat least some arranged to provide backlight illumination to respectivelocal display regions or blocks (108, generally) of the LCD, for examplelocal string 104 for local section or region 108. Thus, environment 100shows lighted panel 102, which may be an LCD, or the like, withcontrolled LED dimming, which may be implemented in accordance withembodiments of the present systems and methods for combined hybrid andlocal dimming. The LCD display system (102) includes an LED backlightsystem (full-array or edge-lit) 104 configured with local display blocksor sections 108 a, 108 b, etc. each illuminated by an associated LEDstring 104 a, 104 b, etc. controlled by driver 106 a, 106 b, etc.

LED driver control unit 110 may be configured to perform hybrid digitalmodulation and hybrid current dimming and illumination control of LEDdrivers 104 of LED backlight system 104, to control LED backlightillumination of (LCD) panel 102. In accordance with embodiments of thepresent systems and methods, hybrid brightness/dimming control is ahybrid of digital modulation control and string current control. Thismay include LED driver control unit 110 providing a master brightnesscontrol signal to digital modulation and current control paths(generally, 112 and 114, respectively, by way of example) for each LEDdriver 106. This master brightness control signal may be generallyassociated with a system/user-desired (global) backlight illuminationfor the LCD, and based on a (user) selected master (percent) brightnesssetting for the (LCD) panel.

In global control of master brightness levels, or the like, LED drivercontrol unit 110 may, for a predefined range of lower LED masterbrightness values, generate a hybrid digital modulation control signalin digital modulation control path 112, which is associated with adigital modulation setting and corresponds to a desired LED brightnessvalue associated with the master brightness control signal. This digitalmodulation may be up to a specified maximum digital modulation setting.Regardless, in accordance with embodiments of the present systems andmethods, string current and string voltage called for by LED drivercontrol unit 110 is maintained substantially constant under such lowerLED master brightness values where hybrid digital modulation control isused for master brightness control of drivers 106 and hence display 102.

Further, in global control of master brightness levels, or the like, LEDdriver control unit 110 may also, for a predefined range of higher LEDmaster brightness values, such as above the LED brightness valueassociated with the maximum digital modulation setting, generate ahybrid current control signal in a current control path 114, whichassociated with a string current corresponding to a desired LEDbrightness value associated with the master brightness control signal.

In accordance with embodiments of the present systems and methods, LEDdriver control unit 110 may also selectively perform local dimming andillumination control for an LED driver (106 a, for example) associatedwith an LED string (104 a, in this example) associated with a selectedlocal display regions or blocks (108 a, for example) of the (LCD) panel102. In accordance with such embodiments, a local LED driver controlunit (116 generally, 116 a in this example), which may be a part of LEDdriver control unit 110 and/or a functionality thereof, may generate alocal digital modulation control signal into the digital modulationcontrol path 112 a, such as via digital modulation control path 118 a inthis example (118, generally in FIG. 1). This local digital modulationsignal is associated with a digital modulation setting corresponding toa desired LED brightness value for selected local display regions orblocks 108 a. The local digital modulation control signal is, inaccordance with embodiments of the present systems and methods,additively or negatively, combined with the master brightness hybriddigital modulation control signal to generate a composite local digitalmodulation control signal along path 112 a (in this example) to be usedto control associated LED driver 106 a (in this example). The compositedigital modulation control signal corresponds to the (total)system/user-desired LED brightness value for selected local displayregions or blocks 108 a (in this example).). Local dimming control isthereby combined with the hybrid digital modulation output of the hybridbrightness/dimming control so that dataflow for local dimming is thesame independent of global/master brightness/dimming control (i.e.,independent of the hybrid brightness/dimming range).

Hybrid dimming control comprises a hybrid combination of PWM and currentdimming control. During a predefined range of relatively lower LEDbrightness levels hybrid dimming control employs PWM modulation tocontrol brightness (with string current substantially constant), and fora predefined range of relatively higher LED brightness levels, PWM isfixed at some maximum setting, such as one-hundred percent, and hybriddimming current control is used to adjust LED brightness by controllingstring current. As noted, such hybrid PWM and current dimming isdescribed in incorporated U.S. Patent Publication 2012/0086701.

FIG. 2 is a graph (200) of (global) hybrid LED dimming as may beemployed as a part of the present systems and methods for combinedhybrid and local dimming control of LEDs, in accordance with someembodiments. Graph 200 plots the output current through the controlledLEDs when an input signal is swept from 0% to 100%. Within first range202 of brightness values, digital modulation control is used, while thecurrent through the LEDs remains relatively constant. Many LEDs havetheir highest optical efficiency, meaning they can generate the highestlumens per watt, when the current through the LEDs is around 25% oftheir rated value (in this case, around 6 mA). For these types of LEDs,that current may be used at lower brightness values, and the actualbrightness of the LEDs may be varied using digital modulation control asshown in FIG. 2. As a result, the LEDs may be operating at or nearmaximum optical efficiency during this time. Within a second range 204of brightness values, current control may be used to adjust the currentthrough the LEDs, while the digital modulation control signal isgenerally above a specified duty cycle (such as 90%) as shown in FIG. 2.During this time, the optical efficiency of the LEDs drops, but the LEDcurrent can increase in order to achieve higher brightness. The LEDcurrent could increase up to a maximum value, such as around 25 mA inthe illustrated example.

In FIG. 2, the separation of the ranges 202 and 204 is made at the point206. This point where the digital modulation control signal generallyreaches a 90-100% duty cycle, and additional increases in brightness arenot achieved by increasing the duty cycle of the PWM control signalsince current is limited to around 6 mA in range 202. Point 206 couldrepresent any suitable brightness value and may vary depending on theLEDs being used. Point 206 could, for instance, represent a brightnessvalue of 20% or 25%. Above point 206, an increase in current is used toachieve higher brightness, and current control is used to adjust thebrightness of the LEDs. By using digital modulation control in lowerbrightness range 202 and current control in higher brightness range 204,significant efficiency gains can be obtained, particularly in lowerrange 202. This can help to reduce power consumption by the LEDs, suchas by 20% or more. This is possible even though the LEDs are producingthe same amount of luminance.

FIG. 3 is a diagrammatic, block illustration of example system orapparatus 300 for employing the present combined hybrid and localdimming control of LEDs, in accordance with some embodiments. Forexample, FIG. 3 may be viewed as a block diagram of (a portion of) anexample (integrated) circuit employing an embodiment of the presentsystems and methods for combined hybrid and local dimming control ofLEDs. That is, apparatus or system 300 may take the form of (a portionof an Integrated Circuit (IC), which may support “pure” PWM dimming andhybrid dimming, such as combined PWM and current brightness control.Such an IC may store and implement LED string calibration data forstring brightness compensation. Also, in such and IC, LED string agingcan be monitored via accumulator registers, or the like, which storeinformation of cumulated brightness for each string.

A brightness control signal path is shown in FIG. 3, wherein LED stringbrightness may be controlled by a base brightness path, which is commonto all LED strings, and an individual brightness setting for each string(i.e. region or local brightness). Typically, a base brightness settingand an individual region setting are multiplied and the end result isused to control the duty cycle of the corresponding LED string current.However, in accordance with the present systems and methods for combinedhybrid and local dimming control of LEDs hybrid dimming function can beenabled for the base brightness control and in addition to this pure PWMdimming, such an IC may support hybrid dimming, which combines PWM andcurrent modes for brightness control. Hybrid dimming can be enabled forthe base brightness path. That is, when hybrid dimming is enabled, itmay control all LED strings. In hybrid mode PWM dimming is used for thelow range of brightness and current dimming for the high brightnesslevels as discussed above and shown in FIG. 2. Current dimming enablesimproved optical efficiency due to increased LED efficiency at lowercurrents. PWM control at low brightness levels ensures smooth andaccurate control. An optional sloper feature may be employed by IC 300to make transition from one brightness value to another smooth.Returning attention to FIG. 3, a diagrammatic block diagram of portion300 of a(n) (integrated) circuit showing example paths for combinedhybrid and local dimming control of LEDs in accordance with someembodiments. Master brightness control 302 may control hybrid dimingcontrol 304, and thereby master control paths 306 and 308 resulting incontrol of master LED backlight hybrid brightness/dimming of a display,or the like.

For example, first master control path 306 may be configured to generatea hybrid digital modulation control signal in digital modulation controlpaths 310 of LED drivers of the display, including local LED stringdriver 312, such as via hybrid dimming control 304. First master controlpath 306 may only generate the hybrid digital modulation control signal,such as via hybrid dimming control 304, in digital modulation controlpath 310 of LED drivers of the display during a predetermined range oflower LED brightness master brightness values provided by masterbrightness control 302. First master control path 306 may also beconfigured to generate hybrid digital modulation control signal, such asvia hybrid dimming control 304, in digital modulation control paths 310of LED drivers 312 of the display up to a specified maximum digitalmodulation setting. Further, first master control path 306 mayadditionally be configured to generate the hybrid digital modulationcontrol signal in digital modulation control paths of LED drivers of thedisplay while maintaining string current and string voltagesubstantially constant, both of which may be carried out by hybriddimming control 304, or the like.

Second master control path 308 may be configured to generate a hybridcurrent control signal, such as via hybrid dimming control 304, instring current control paths 314 of the LED drivers of the display,including local LED string driver 312. Second master control path 308may be configured to generate the hybrid current control signal instring current control paths of the LED drivers of the display so as tocorrespond to a system/user-desired LED brightness value associated withthe master brightness control signal, such as from master brightnesscontrol 302. For example, hybrid dimming control 304, as part of secondmaster control path 308, may be configured to generate the hybridcurrent control signal during a predetermined range of higher LED masterbrightness values, such as may be dictated by master brightness control302. Further in this regard, second master control path generation ofthe hybrid current control signal in string current control paths may befor string brightness above a LED brightness value associated with amaximum digital modulation setting (such as provided via first mastercontrol path 306.

Thus, LED driver 312 may be operable as a current sink, which supportsboth high resolution hybrid PWM control and hybrid current (amplitude)control. Advantages of each technique include LED backlight powersaving. Further savings for local or regional dimming control may beobtained by employing PWM control combined with this hybrid PWM andcurrent control in accordance with embodiments of the present systemsand methods.

Local dimming control path 316 may be configured to selectively performlocal dimming control of LED driver 312, which is associated with aselected LED string associated with a selected local display regions orblocks of the display. Local control path 316 may be configured togenerate a local digital modulation control signal, such as by localstring brightness control 318, in the digital modulation control path(320, 322, 324) of LED driver 312 associated with the selected localdisplay regions or blocks. That is, local control path 316 may beconfigured to generate the local digital modulation control signal intothe digital modulation control path (320, 322, 324) that is associatedwith a digital modulation setting of LED driver 312 associated with theselected local display regions or blocks. Local control path 316,particularly local string brightness control 318, may be configured togenerate a local digital modulation control signal corresponding to adesired LED brightness value for the selected local display regions orblocks into the digital modulation control path (320, 322, 324) of LEDdriver associated with that selected local display regions or blocks.

Local control path 316 may be further configured to combine the localdigital modulation control signal from path 320 with the hybrid digitalmodulation control signal from path 210, such as by multiplier 326 togenerate a composite local digital modulation control signal, output onpath 322, for controlling digital modulation (PWM) generation, such asat PWM generator 328 for provision to local LED driver 312, such as viapath 324. The composite local digital modulation control signal outputfrom multiplier 326, via path 322, preferably corresponds to asystem-desired LED brightness value for the selected local displayregions or blocks such as may be imparted by local LED driver 312 inresponse to the PWM signal provided by PWM generation at 322, via path324. As a result, local dimming control is combined with the hybriddigital modulation output of the hybrid brightness/dimming control suchthat dataflow for local dimming is the same independent of global/masterbrightness/dimming control (i.e., independent of the hybridbrightness/dimming range).

Thus in operation, such as depicted in method implementation 400 of FIG.4, which is a flowchart of example implementation 400 of the presentsystems and methods for combined hybrid and local dimming control ofLED, according to some embodiments. Therein, overall brightness of anLED backlight panel may be controlled, at least in part, by globalhybrid brightness/dimming control of a plurality of strings of backlightLEDs making up a backlight system as a hybrid of digital modulation andcurrent dimming at 402.

As part of this global diming, such as during relatively lower LEDmaster brightness levels, hybrid digital modulation (PWM) dimming may beperformed at 404. This PWM control, or the like, may control brightnessup to a specified maximum digital modulation setting and may result incurrents for individual LED strings (such as established at 406 below)remaining substantially constant.

Hybrid current dimming control, such as may be performed for relativelyhigher master LED brightness levels, may adjust LED string current at406, with hybrid digital modulation (such as established at 404) fixedat the maximum digital modulation setting.

As noted, local dimming can be implemented in an LCD display, or thelike with an LED backlight system configured into local display regionsor blocks in accordance with embodiments of the present systems andmethods for combined hybrid and local dimming control of LED. Thebacklight LEDs are positioned so that an LED string illuminates anassociated local display block/region. Each LED driver output can have aseparate brightness setting controlling the illumination of itsassociated LED string. Local dimming control is used to enable dimmingcontrol of each local display block/region independent of dimmingcontrol applied to the rest of the display. In example embodiments ofcombined hybrid and local dimming (brightness) control according toaspects of the present systems and methods, “global” hybrid (e.g. PWMand current) illumination and dimming control, such as may be based onmaster brightness control, is applied to the full display, based on thismaster brightness control, As described above with respect to steps 402through 406, and is combined with local illumination and dimming controlfor local display blocks or regions, as described below. Localillumination and dimming is implemented by introducing local dimming PWMmodulation into the PWM control path.

Local dimming control of each string of LEDs may be performed byproducing a local (PWM) digital modulation signal at 408 and introducingthe local (PWM) digital modulation signal into a hybrid digitalmodulation control path for each string at 410, correspondingly changingdigital modulation for the string to be a combination of hybrid digitalmodulation and local digital modulation. In accordance with embodimentsof the present systems and methods, introducing this local PWM signal,or the like at 410 into the hybrid digital modulation from 404 for eachstring, without affecting string voltage or string current, such as setat 406.

Thusly, local PWM control is combined with the hybrid base PWM controlsettings (i.e. common settings), into a composite PWM used to controlthe associated LED driver. This combined hybrid and local dimming andbrightness control, including composite hybrid and local PWM control,can be implemented without affecting string currents or string voltagesotherwise set by the global hybrid dimming control (at 406). That is,for a local display block or section, local dimming control is basedonly on PWM modulation (from 404 and 408, combined). String current issubstantially constant for all LED strings, as determined by the globalhybrid PWM and current dimming control, based on a master brightnesssetting. Thereby, local dimming control may be combined with the hybriddigital modulation output of the hybrid brightness/dimming control suchthat dataflow for local dimming may be the same independent ofglobal/master brightness/dimming control (i.e., independent of thehybrid brightness/dimming range).

Advantages of combined hybrid and local dimming control according toaspects of the present systems and methods include the LED stringvoltages remaining substantially constant during local dimming, whichonly occurs during hybrid PWM dimming control, so that LED driverheadrooms remain substantially constant. The present systems and methodsalso avoid other alternatives, such as separate power supplies for eachof the strings, or different headrooms for the LED current sinks Powerefficiency is optimized, since, during local dimming, LED driverheadrooms remain substantially constant, whereas modulating the currentfor each string would result in different headroom voltages, since theLED voltage is a function of current. LED string fault detection canwork as normally implemented in LED driver ICs, because string voltagesare the same. Further, a DC-to-DC converter, such as a Boost converter,providing supply voltage for the LED(s) (strings) may operate at asubstantially constant voltage, even with local dimming that varies fromframe to frame, because with hybrid PWM modulation, string voltage issubstantially constant (i.e. LEDs have substantially constant voltageeven when optical output changes).

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions,and the associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

What is claimed is:
 1. A method comprising: performing global hybriddimming control of a plurality of strings of backlight light emittingdiodes making up a backlight system as a hybrid of digital modulationdimming and current dimming, comprising: performing hybrid digitalmodulation dimming to control brightness up to a specified maximumdigital modulation setting; and performing hybrid current dimmingcontrol, with hybrid digital modulation fixed at the maximum digitalmodulation setting, to adjust a string current for each of the stringsof backlight light emitting diodes; and performing local dimming controlof one or more of the strings of backlight light emitting diodes byintroducing a local digital modulation signal into a hybrid digitalmodulation control path for each of the one or more strings of backlightlight emitting diodes, correspondingly changing digital modulation forthe string to be a combination of hybrid digital modulation and localdigital modulation.
 2. The method of claim 1, wherein performing hybriddigital modulation dimming to control brightness up to the specifiedmaximum digital modulation setting further comprises maintaining stringcurrents substantially constant.
 3. The method of claim 1, whereinperforming hybrid digital modulation dimming to control brightness iscarried out during a predefined range of relatively lower light emittingdiode master brightness levels.
 4. The method of claim 1, whereinperforming hybrid current dimming control is carried out to adjust lightemitting diode string current for a predefined range of relativelyhigher master light emitting diode brightness levels.
 5. The method ofclaim 1, wherein introducing the local digital modulation signal intothe hybrid digital modulation control path for each string is carriedout without affecting string current or string voltage.
 6. The method ofclaim 1, wherein the digital modulation is pulse width modulation.
 7. Anapparatus comprising: master control paths to control master lightemitting diode backlight illumination of a display, comprising: a firstmaster control path configured to generate a hybrid digital modulationcontrol signal in digital modulation control paths of light emittingdiode drivers of the display; a second master control path configured togenerate a hybrid current control signal in string current control pathsof the light emitting diode drivers of the display; and a local controlpath to selectively perform local dimming control of a light emittingdiode driver associated with a selected light emitting diode stringassociated with a selected local display region of the display, thelocal control path to generate a local digital modulation control signalin the digital modulation control path of the light emitting diodedriver associated with the selected local display region and to combinethe local digital modulation control signal with the hybrid digitalmodulation control signal to generate a composite local digitalmodulation control signal, controlling the light emitting diode driverassociated with the selected local display region.
 8. The apparatus ofclaim 7, wherein the first master control path is operable to generatethe hybrid digital modulation control signal in digital modulationcontrol paths of light emitting diode drivers of the display during apredetermined range of lower light emitting diode brightness masterbrightness values.
 9. The apparatus of claim 7, wherein the first mastercontrol path is operable to generate hybrid digital modulation controlsignal in digital modulation control paths of light emitting diodedrivers of the display up to a specified maximum digital modulationsetting.
 10. The apparatus of claim 7, wherein the first master controlpath is operable to generate the hybrid digital modulation controlsignal in digital modulation control paths of light emitting diodedrivers of the display while maintaining string current and stringvoltage substantially constant.
 11. The apparatus of claim 7, whereinthe second master control path is operable to generate the hybridcurrent control signal in string current control paths of the lightemitting diode drivers of the display during a predetermined range ofhigher light emitting diode master brightness values.
 12. The apparatusof claim 7, wherein the second master control path is operable togenerate the hybrid current control signal in string current controlpaths of the light emitting diode drivers of the display above the lightemitting diode brightness value associated with the maximum digitalmodulation setting.
 13. The apparatus of claim 7, wherein the secondmaster control path is operable to generate the hybrid current controlsignal in string current control paths of the light emitting diodedrivers of the display to correspond to a desired light emitting diodebrightness value associated with the master brightness control signal.14. The apparatus of claim 7, wherein the local control path is furtheroperable to generate the local digital modulation control signal in thedigital modulation control path that is associated with a digitalmodulation setting of the light emitting diode driver associated withthe selected local display region.
 15. The apparatus of claim 7, whereinthe local control path is further operable to generate the local digitalmodulation control signal in the digital modulation control path of thelight emitting diode driver associated with the selected local displayregion corresponds to a desired light emitting diode brightness valuefor the selected local display region.
 16. The apparatus of claim 7,wherein the composite local digital modulation control signalcorresponds to a desired light emitting diode brightness value for theselected local display region.
 17. The apparatus of claim 7, wherein thedigital modulation is pulse width modulation.
 18. A liquid crystaldisplay system with controlled light emitting diode dimming, comprising:a plurality of strings of backlight light emitting diodes arranged toprovide backlight illumination to the liquid crystal display, eachcontrolled by an light emitting diode driver, and at least some arrangedto provide backlight illumination to respective local display regions ofthe liquid crystal display; a local light emitting diode driver controlunit to: generate, for a predefined range of range of lower lightemitting diode master brightness values, a hybrid digital modulationcontrol signal in a digital modulation control path associated with adigital modulation setting corresponding to a desired light emittingdiode brightness value associated with a master brightness controlsignal, up to a specified maximum digital modulation setting, whilemaintaining string current and string voltage substantially constant;generate, for a predefined range of higher light emitting diode masterbrightness values above the light emitting diode brightness valueassociated with the maximum digital modulation setting, a hybrid currentcontrol signal in a current control path associated with a stringcurrent corresponding to a desired light emitting diode brightness valueassociated with the master brightness control signal; and selectivelyperform local dimming and illumination control for a light emittingdiode driver associated with a light emitting diode string associatedwith a selected local display region of the liquid crystal display bygenerating a local digital modulation control signal in the digitalmodulation control path associated with a digital modulation settingcorresponding to a desired light emitting diode brightness value for theselected local display region, and combining the local digitalmodulation control signal with the hybrid digital modulation controlsignal to generate a composite local digital modulation control signalused to control the associated light emitting diode driver, thecomposite digital modulation control signal corresponding to the desiredlight emitting diode brightness value for the selected local displayregion.